1. Field of the Invention
The present invention relates to a high-frequency probe having a signal line which has a fore end pressed against a signal electrode of a device-under-test (abbreviated to DUT hereafter) to be measured, and a rear end connected to a connector for connection to an external measuring instrument.
The present invention especially relates to a high-frequency probe for use in measurement of a DUT, which is placed on a device stage establishing electrical connection with a ground electrode of the DUT and serving as a ground electrode and which has a number of signal electrodes arrayed with a narrow pitch. More particularly, the present invention relates to a tip portion structure of a high-frequency probe and a method of fabricating a probe tip portion, which can provide contact with the signal electrodes and electrical characteristics with higher reliability and more stability.
2. Description of the Related Art
Hitherto, as illustrated in FIGS. 1A and 1B, a high-frequency probe 100 of the above-mentioned type comprises a body block 110, a tip portion 120, and a connector 130. A coaxial cable 111 penetrating the body block 110 is connected to the connector 130 for electrical connection which connects an external measuring instrument and the tip portion 120 brought into contact with signal electrodes of a DUT to be measured.
Further, as illustrated in FIG. 2, the tip portion 120 comprises a signal contact lead 121 and two ground contact leads 122, each of which has resiliency. The ground contact leads 122 are arranged side by side on both sides of the signal contact lead 121 and on substantially the same plane normal to a direction in which the conductors bend due to resiliency. Thus the signal contact lead 121 and the ground contact leads 122 are formed in a coplanar structure.
Usually, the signal contact lead 121 at the center serves as a contact lead for a signal and is brought into contact with a signal electrode 211 of a DUT 210. On the other hand, the ground contact leads 122 on both sides of the signal contact lead 121 serve as ground contact leads and are brought into contact with ground electrodes 212 of the DUT 210.
In case that the probe tip portion has such a conductor array structure, the DUT is limited to a coplanar type device wherein signal electrodes and ground electrodes are arranged on the same plane and with the same pitch as conductors arranged in a tip portion of a high-frequency probe.
A large surface area is required in the device of the above-mentioned type having two ground electrodes arranged on both sides of one signal electrode and on the same plane. For compound devices obtained from a wafer of gallium arsenide (GaAs), in particular, the wafer cost is higher than that of a silicon wafer. Therefore, a reduction in the number of devices obtained from one piece of wafer considerably pushes up the device cost. Accordingly, a mass-produced device is constructed such that ground electrodes are not disposed on the same plane as a signal electrode, and uses its backside surface as a ground electrode. In addition, a chip area is reduced and a wafer thickness is thinned to cut down the device cost and to ensure a desired high-frequency characteristic.
In a case that the conventional high-frequency probe described above is employed to measure a DUT of such a structure that the backside surface entirely serves as a ground electrode, any contact between electrodes of the DUT and contact leads of a probe tip portion cannot be achieved. Accordingly, the measurement is performed for the DUT mounted on a board. In this case, the board has measuring electrodes arranged with the same pitch as the contact leads of the high-frequency probe, and the high-frequency probe can be connected to the board.
Also, in the probe having the above tip portion structure, pressing forces are applied to the electrodes of a DUT in an unstable condition because the probe contact leads are pressed against the DUT electrodes with any one electrode serving as a fulcrum. Such an unstable condition may damage the contact lead ends of the probe due to application of an excessive pressure.
The conventional high-frequency probe described above has therefore problems as follows.
The first problem is that the measurement is very difficult or impracticable when the signal electrode and the ground electrodes of the DUT to be measured are not arranged on the same plane.
The reason is because the contact leads of the probe are arranged side by side on the same plane for making contact with the DUT electrodes. Further, because the contact leads of the probe has the pitch in match with the array pitch of those DUT electrodes, the contact leads cannot contact with DUT electrodes having other structures not in match with that pitch.
The second problem is that, in a case of the DUT not having a coplanar structure, a measuring board must be prepared and the measurement requires time and labor.
The reason is because the above-described high-frequency probe has the signal contact lead and the ground contact leads which are of the coplanar structure. In other words, for a measuring DUT of any structure different from the coplanar type, a measuring board is necessary and the DUT being measured requires to be mounted and dismounted to and from the measuring board. For the DUT having a structure wherein a number of signal electrodes are arrayed with a narrow pitch, particularly, a lot of time and labor are taken for wiring job.
The third problem is that a sufficient contact pressure is not obtained in a case that the contact lead of the probe is pressed against the electrode of the DUT for measurement. Thus resulting is in instability in measurement of electrical characteristics, and the contact lead of the probe is susceptible to damage.
The reason is because the above-described high-frequency probe has the structure wherein the contact lead contacts the signal electrode of the DUT under measurement and bends at a freely-suspended end. Also, because a pressing force is exerted on the contact lead of the probe to bend its end about a fulcrum positioned on the contact lead, it is difficult to adjust the pressing force. Stated otherwise, the pressing force must be somewhat moderated in view of such a risk that damage may occur at the end of the contact lead if the pressing force is intensified to make stable measurement.
The fourth problem is that the DUT has an increased area and the product cost is increased.
The reason is because, for measuring a DUT by the above-described high-frequency probe, ground electrode of the DUT requires to be arranged on both sides of a signal electrode thereof on the same plane in the same positional relationship as that between a signal contact lead and ground contact leads of the probe. In other words, because a surface area of the DUT is increased, the number of DUTs produced from one piece of wafer is reduced. The fourth problem is particularly remarkable in a case that the DUT is a compound device of gallium arsenide being more expensive than silicon.
Meanwhile, U.S. Pat. No. 5,506,515 discloses a simplified structure of the tip portion of the high-frequency probe of the above-described type. The disclosed structure of the tip portion of the high-frequency probe is illustrated in FIG. 3. In the figure, a coaxial cable 140 has a cross section surface at its end and comprises a coaxial inner conductor, a coaxial outer conductor, and a dielectric interposed between both the conductors, which are in a concentric relation.
Specifically, the coaxial cable 140 comprises three concentric parts, i.e., a coaxial inner conductor 141 at the axial center, a coaxial outer conductor 142 at an outer periphery, and a dielectric 143 interposed between both the conductors 141 and 142. The end of the coaxial cable 140 is cut perpendicularly to the coaxial direction to provide a cross section portion 144. A central contact lead 151 is fixedly connected to the coaxial inner conductor 141, while outer contact leads 152 are positioned on both sides of the central contact lead 151 and are fixedly connected to the coaxial outer conductor 142.
A description will now be made on the tip portion structure of the high-frequency probe of the above-described type and a method fabricating the probe tip portion with reference to FIG. 4A to FIG. 4D in addition to FIG. 3. FIG. 4A to FIG. 4D are bottom views looking, from the back side, the probe tip portion illustrated in the perspective view of FIG. 3 and illustrating one example of successive fabricating steps.
First, FIG. 4A shows a state after a step of cutting the coaxial cable 140 in a plane normal to the axial direction to form the cross section portion 144.
Then, FIG. 4B shows a state after a step of cutting out a semi-cylindrical portion from the end of the coaxial cable 140 along a plane containing the axis and a plane perpendicular to that plane. Thus defining is a longitudinal cut surface 145 containing the axis, and a half cross section 146 perpendicular to the longitudinal cut surface 145.
Thereafter, in a step illustrated in FIG. 4C, a frame component 150 is positioned on and fixedly connected to the longitudinal cut surface 145. The frame component 150 is formed by machining together with the central contact lead 151, the outer contact leads 152, and a base plate 153. The base plate 153 supports those leads such that the outer contact leads 152 are positioned on both sides of the central contact lead 151. And the outer contact leads 152 are connected to the coaxial outer conductor 142 in the longitudinal cut surface 145 in a state that the central contact lead 151 is connected to the coaxial inner conductor 141 in the longitudinal cut surface 145.
Finally, in a step illustrated in FIG. 4D, the base plate 153 is no longer needed and is cut off from the contact leads, whereby the tip portion structure illustrated in FIG. 3 is completed.
The above method of fabricating the tip portion structure of the conventional high-frequency probe requires the frame component including the contact leads in addition to the coaxial cable. The frame component includes one central contact lead, two outer contact leads, and a base plate. Therefore, the above method requires the steps of fixedly connecting the one central contact lead to one coaxial inner conductor of the coaxial cable and the two outer contact leads to one coaxial outer conductor thereof, respectively, and then cutting off the base plate from the contact leads. In other words, the frame component in the preparatory step has a complicated shape, and the completed tip portion has a relatively large number of parts. This raises a problem that the fabricating process is complicated and the product cost is increased.
Otherwise, the DUT is downsized and has a large number of signal electrodes arrayed with a narrow pitch and a ground electrode brought into contact with a device stage serving as a test or measurement stage. In this case, a tip portion of a high-frequency probe adapted for such a DUT can also be fabricated by using a coaxial cable and a frame component with contact leads and applying the fabricating method described above. A similar problem as described above however still remains.
It is an object of the present invention to provide a tip portion structure which is useful as a high-frequency probe and which can solve the above-described problems.
It is another object of the present invention to provide a method of fabricating a tip portion structure as described above in a very simple manner.
According to an aspect of the present invention, a tip portion structure of a high-frequency probe to which the present invention is applicable has a signal line which has a fore end pressed against a signal electrode of a DUT (device-under-test) being placed on a device stage, and a rear end connected to a connector for connection to an external measuring instrument. The tip portion structure comprises a tip substrate having a front surface and a back surface attached to said signal line formed on the back surface, a conductive thin ground plate covering entirely the front surface of the tip substrate, a plate spring positioned to apply a pressure to the tip substrate in a state that the fore end of the signal line is pressed against the signal electrode of the DUT, and a conductive ground block positioned with a predetermined gap against the back surface of the tip substrate, and contacting with a ground surface of the device stage to establish electrical connection in a state that the fore end of the signal line is pressed against the signal electrode of the DUT.
According to another aspect of the present invention, a method is for use in fabricating a tip portion of a high-frequency probe formed of a coaxial cable comprising a coaxial inner conductor, a coaxial outer conductor, and a dielectric interposed between the coaxial inner conductor and the coaxial outer conductor in a concentric relation. The method comprises forming a cross section surface by cutting the coaxial cable at a plane perpendicular to the axial direction of the coaxial cable, forming a oblique cut surface by cutting the cross section surface from substantially the center thereof along at least one oblique plane with respect to the axial direction of the coaxial cable, fixing a ring made of a conductive material over a periphery of the coaxial outer conductor to establish electrical connection with the coaxial outer conductor, and bonding a contact bump to the coaxial inner conductor exposed in the cross section surface. The fixing of ring and the bonding contact bump are executed one after the other in this order or in the reversed order.